Light sources which are to be combined with optical systems, and especially for use in projection systems, should be as close to point sources as possible. Additionally, the light distribution should be spectrally as homogeneous as possible, to ensure concentrated light suitable for color projection. The spectral composition of the light is also a quality criterion for the lamp. There must be appropriate and sufficient proportions of the basic spectral colors blue, green and red to ensure good color rendition; the balance of the spectral components should be appropriate for the desired color temperature. The lifetime of the lamp is of importance, as well as the aging factors, also known as maintenance factors, that is, changes in the light intensity or light flux, color temperature and spectral distribution which, during the lifetime of the lamp, should be as small as possible.